The effects of alloy microstructure refinement on the short-term thermal oxidation of NiCoCrAlY alloys

Three γ + β NiCoCrAlY alloys (a cast alloy, a laser-surface-melted (LSM) alloy, and a coating as deposited by electron beam-physical vapor deposition (EB-PVD)) with similar average composition (Ni-20Co-19Cr-24Al-0.2Y in at. pct), but with different microstructures prior to oxidation, were oxidized for 0.5 and 1 hours at 1373 K in an Ar 20 vol pct O2 atmosphere (i.e., at a partial oxygen pressure of 20 kPa). It was found that on the alloy with β precipitates larger than 20 µm, the oxide layer was nonuniform in thickness, and had a laterally inhomogeneous composition and phase constitution. In this case, the oxide layer developed on top of the γ phase was thicker than that formed on top of the β phase and consisted of a NiCr2O4/Cr2O3 outer and an α-Al2O3 inner layer. For the thinner oxide formed on top of the β phase, the outer layer was constituted of a Cr and Co containing NiAl2O4 spinel and the inner layer also consisted of α-Al2O3. For the alloys with β precipitates smaller than 3 µm, a uniform and laterally homogeneous oxide formed, consisting of a Cr and Co containing NiAl2O4 outer layer on top of an α-Al2O3 inner layer. After oxidation, Y was distributed as numerous, small precipitates within the oxide layer for a homogeneous Y distribution prior to oxidation, or as a few, very large pegs along the γ/β phase boundaries of the alloy for an inhomogeneous Y distribution prior to oxidation. The performance of the alloys upon thermal cycling was improved for smaller β precipitates and for a more homogeneous Y distribution in the alloy prior to oxidation.